Mandibular position is an important parameter used for the diagnosis of dentofacial deformities, as well as for orthognathic surgery planning and execution. Centric relation (anterior and superior relationship of the mandibular condyles interposed by the thinnest portion of their disks against the articular eminencies), centric occlusion (when lower teeth contact upper teeth at centric relation), and maximal intercuspation (complete interdigitation of lower and upper teeth) are not often addressed as factors that influence the results of orthognathic surgery, although these relationships are critical to ensure accuracy during the surgery. The present study assessed occlusal measurements taken before and after the induction of general anaesthesia from consecutive orthognathic surgery subjects. The variables assessed included the differences between these occlusal measurements, patient age, gender, type of deformity, and type of proposed orthognathic surgical procedure. The results demonstrated statistically significant differences for mandibular retrusion from maximal intercuspation to centric occlusion position, whereas the mandible appeared not to change significantly from centric occlusion after the induction of general anaesthesia. Patient age and the type of deformity appeared to influence the results. While in most instances centric occlusion can be adequately reproduced under general anaesthesia, for some specific orthognathic cases more accurate results might be obtained if the mandible-first sequence is used.
The mandible and temporomandibular joint (TMJ) apparatus is a complex structure commonly used by dentists and dental specialists as an anatomical reference in a number of different clinical situations. The complexity is evident in the anatomy of the mandible with severe bends and curves and the presence of foramina, the intrinsic relationship of the mandible to the surrounding structures, the complex opening and closing jaw functions relative to the masticatory muscles, and the presence of a pair of independent but connected (to each other) articulations to the cranial base through the TMJ.
Since the mandible provides the osseous support for the mandibular dentition, its range of motion allows dental occlusion to adapt to functional requirements such as mastication, speech, and swallowing. Mandibular position can be defined at the level of the TMJ as centric relation (CR), which corresponds to an anterior and superior relationship of the mandibular condyles interposed by the thinnest avascular portion of the respective disks against the articular eminencies of the temporal bone. At the level of the dentition, mandibular position can be defined as centric occlusion (CO) when the lower teeth contact the upper teeth with the TMJ positioned in CR. Finally, maximal intercuspation (MI) is defined as the complete interdigitation of the lower and upper teeth, not necessarily coinciding with CO, independent of any reference to the position of the TMJ.
Malocclusions are usually the result of dental misalignment, which can be corrected by orthodontic treatment. However, in some instances, the dental malposition may reflect discrepancies of the bony support structures including the maxilla and mandible. In such cases, orthognathic surgery is a commonly used surgical procedure that enables the correction of skeletal discrepancies, allowing orthodontic therapy to correct the discrepancies in tooth alignment.
Mandibular position is an important parameter assessed in the diagnosis of dentofacial deformities, as well as for the planning and execution of orthognathic surgery. In order to determine mandibular position, CO is used to observe the difference between the maxillary and mandibular teeth clinically, and to mount the maxillary and mandibular dental cast models for evaluation and in order to perform model surgery. During the surgical procedure, it is essential to have the mandible in the CO position in order to allow the actual surgery to correspond to the model surgery, especially for bimaxillary surgical cases. It should be noted that some authors have shown that the mandible may assume a different position to that which was recorded presurgery when the patient is under general anaesthesia (GA) in a supine position, and that this results in an altered CO position; however, few studies have documented this phenomenon clinically.
The present study aimed to address the following questions among patients with dentofacial deformities undergoing orthognathic surgery: (1) What is the difference between occlusal registrations obtained in the CO and MI positions? (2) Is there a difference in CO and MI positions obtained in the awake and upright patient position and in the supine position under GA? Does the patient age, gender, type of dentofacial deformity, or type of surgical procedure influence any of the above measurements and relationships?
Materials and methods
The clinical data were collected prospectively from consecutive patients who underwent orthognathic surgery in the department of oral and maxillofacial surgery of the study hospital between July 2012 and April 2013.
The following variables were evaluated: gender (male or female), age (categorized into three groups according to the statistical distribution of the study group: ≤24 years, 25–39 years, >39 years), type of dentofacial deformity (class II or class III), and type of orthognathic surgery performed (single-jaw (maxilla or mandible) or double-jaw (maxilla and mandible)).
The inclusion criteria were the following: age ≥18 years, overall good systemic health (American Society for Anesthesiologists (ASA) classification I or II), cognitive understanding of the orthognathic surgery to be performed, and voluntary consent of acceptance to be enrolled in the proposed research project. Exclusion criteria were concomitant or prior TMJ surgery, absence of mandibular central incisors, absence of at least one mandibular molar on each side, and refusal to participate in the research project.
After a discussion including a comprehensive explanation of the proposed surgical procedure as well as the methodology of the research protocol, patients who agreed to participate signed an informed consent agreement. Three oral and maxillofacial surgeons performed all of the surgical procedures as part of work for the Brazilian public health system.
This study was performed in accordance with the principles of the Declaration of Helsinki and is registered at ClinicalTrials.gov under the identifier NCT01486069 . Ethics committee approval was obtained.
Dental occlusion registration
Dental occlusion was registered for three distinct clinical positions: MI in a semi-supine position, and CO in a semi-supine position and in a supine position while under GA. Condensation silicone was used as the impression material (Silon2 APS; Dentsply, Santiago, Chile). The same examiner was responsible for performing each MI and CO bite registration for all patients at all time points.
For MI registration, the patient was positioned in a dental chair in a semi-supine fashion (inclined at an angle of 45° to the horizontal plane) and voluntary maximal dental occlusion was observed twice without the use of impression material. Just prior to the third attempt at maximal occlusion, impression material was placed over the mandibular teeth and the patient was instructed to bite in a fashion similar to the first two attempts.
For CO registration, the patient was positioned again in a semi-supine fashion. The patient was asked to relax and position his/her tongue gently against the palate and to maintain it in that position; then, gentle manual pressure was applied to the chin in order to achieve the most anterior and superior position of the mandibular condyles, moving the mandible until the first contact of opposing teeth was noted. This technique was performed twice without any registration; just prior to the third attempt, impression material was placed over the mandibular teeth while the jaw manipulation was repeated and the bite registration was performed.
Occlusal registration under GA was accomplished with the patient in a supine position immediately following the induction of GA (propofol and fentanyl) and nasotracheal intubation (preceded by the use of atracurium as the neuromuscular blocking agent). This particular time was chosen since the influence of the muscles of mastication would be minimized or eliminated with muscular paralysis. The tongue was carefully positioned towards the posterior hard palate, and gentle pressure was applied to the chin in order to achieve the most anterior and superior position of the mandibular condyles, with movement of the mandible until the first contact of opposing teeth was noted. This technique was performed twice without any bite registration; during the third attempt, impression material was placed over the mandibular teeth and the technique repeated and the bite registration was performed. Pharyngeal packing with gauze was performed only after the occlusal registration process had been completed.
Measurement of mandibular position
Each patient had two maxillary and two mandibular dental cast models, which were mounted on a semi-adjustable articulator (model 4000-S; Bio-Art, São Carlos SP, Brazil) based upon an organized mounting sequence. The face-bow was levelled to make it parallel to the interpupillary plane (axis–orbital plane), and the forehead support was used as a three-dimensional reference to mount the first maxillary cast on the articulator. The CO registration was then used to mount the first mandibular cast model; next, the first maxillary model was removed and the second maxillary model was mounted over the same first mandible. Following this, the first mandibular model was removed and the second mandibular model was mounted under the second maxillary model using the MI registration. The mounting of the maxillary models was checked for accuracy, and if positive, orthognathic model surgery was ready to be performed.
Dental anatomical references were marked on the mandibular models in the anterior region (mesio-incisal angle of the left central incisor), the right posterior region (distobuccal cusp of the most posterior right molar), and the left posterior region (distobuccal cusp of the most posterior left molar) ( Fig. 1 ). Subsequently, the casts were measured with an electronic calliper attached to a granite platform (Erickson Model Block). For the anteroposterior (AP) axis, measurements were obtained from the three reference marks in the anterior region and the right and left sides with the posterior region of the mandibular model facing down. For the vertical (V) axis, measurements were obtained from the same three points with the base of the mandibular model facing down. For the transverse (T) axis, measurements were obtained only from the anterior region with the left side of the mandibular model facing down ( Fig. 2 ).
Model surgery for single-jaw procedures was done using a Galetti articulator. Double-jaw procedures required model surgery that was performed according to a pre-determined sequence : the maxillary model was placed in the desired final position, the second mandibular model (mounted on MI) was then set into occlusion with the ‘operated’ maxilla thus providing a final occlusion and allowing fabrication of the final splint; the ‘operated’ mandibular model was left in place while the ‘operated’ maxillary model was removed and replaced by the unoperated maxillary model, thus providing an intermediate occlusal relationship and allowing the fabrication of the intermediate splint. All double-jaw procedures were planned to have the surgery begin with the mandible-first sequence.
After surgery, a mandibular model was remounted against the unoperated maxillary model, this time using the GA registration and allowing the proposed measurements to be performed. Considering that different individuals possess variable mandibular dimensions, the absolute measurement was not recorded, but rather a mathematical equation was used to determine the difference between two distinct moments for the same patient: the difference between CO and MI (CO–MI) and the difference between GA and CO (GA–CO). Hence, the interpretation of the data was accomplished according to the determinations for the AP, V, and T axes as detailed below.
AP: positive values for CO–MI indicated that the mandible was found in a more anterior position at CO when compared to MI; for the GA–CO measurement, positive values indicated that the mandible was in a more anterior position at GA when compared to CO. Negative values, on the other hand, indicated that the mandible was in a more posterior position at CO when compared to MI; for the GA–CO measurement, negative values indicated that the mandible was in a more posterior position at GA when compared to CO ( Fig. 2 a).
V: positive values for CO–MI indicated that the mandible was found in a more superior position at CO when compared to MI; for the GA–CO measurement, positive values indicated that the mandible was in a more superior position at GA when compared to CO. Negative values, on the other hand, indicated that the mandible was in a more inferior position at CO when compared to MI; for the GA–CO measurement, negative values indicated that the mandible was in a more inferior position at GA when compared to CO ( Fig. 2 b).
T: positive values for CO–MI indicated that the mandible was found deviated to the right at CO when compared to MI; for the GA–CO measurement, positive values indicated that the mandible was found deviated to the right at GA when compared to CO. Negative values, on the other hand, indicated that the mandible was found deviated to the left at CO when compared to MI; for the GA–CO measurement, negative values indicated that the mandible was found deviated to the left at GA when compared to CO ( Fig. 2 c).
Data were analyzed descriptively using SPSS 15.0 (SPSS Inc., Chicago, IL, USA). The results of the measurements were compared for the proposed variables (age, gender, type of deformity, type of surgery) by bivariate comparison using a Student’s t -test or by analysis of variance (ANOVA) associated with the Tukey test; statistical significance was set at P < 0.05.
The total study group comprised 30 patients (mean age 30.5 years; median age 29.5 years). Patient age was categorized into three groups according to the incidence of distribution of the sample: ≤24 years, 25–39 years, and >39 years. The distribution of gender, age, type of deformity, and type of surgery is detailed in Table 1 .
|Variable||Group||Number of patients ( n )|
|Patient age||≤24 years||8|
|Type of deformity||Class II||16|
|Type of orthognathic surgery||Single-jaw||14|